Evaluation of left ventricular function is of interest for both diagnostic and therapeutic applications. During normal cardiac function, the atria and ventricles observe consistent time-dependent relationships during the systolic (contractile) phase and the diastolic (relaxation) phase of the cardiac cycle. During cardiac dysfunction associated with pathological conditions or following cardiac-related surgical procedures, these time-dependent mechanical relationships are often altered. This alteration, when combined with the effects of weakened cardiac muscles, reduces the ability of the ventricle to generate contractile strength resulting in hemodynamic insufficiency.
Ventricular dyssynchrony following coronary artery bypass graft (CABG) surgery is a problem encountered relatively often, requiring post-operative temporary pacing. Atrio-biventricular pacing has been found to improve post-operative hemodynamics following such procedures.
Chronic cardiac resynchronization therapy (CRT) has been clinically demonstrated to improve indices of cardiac function in patients suffering from congestive heart failure. Cardiac pacing may be applied to one or both ventricles or multiple heart chambers, including one or both atria, to improve cardiac chamber coordination, which in turn is thought to improve cardiac output and pumping efficiency. Clinical follow-up of patients undergoing resynchronization therapy has shown improvements in hemodynamic measures of cardiac function, left ventricular volumes, and wall motion. However, not all patients respond favorably to cardiac resynchronization therapy. Physicians are challenged in selecting patients that will benefit and in selecting the optimal pacing intervals applied to resynchronize the heart chamber contractions.
Selection of atrial-ventricular (A-V) and inter-ventricular (V—V) pacing intervals are often based on echocardiographic studies performed to determine the settings resulting in the best acute hemodynamic response. Significant hemodynamic changes may not always be acutely observable in an individual patient, however, using non-invasive monitoring methods. Selection of parameters may therefore be based on avoidance of altered or impeded ventricular filling. In the MIRACLE clinical trial conducted to evaluate cardiac resynchronization therapy, as understood by the inventor, the A-V interval was optimized individually in patients by shortening the A-V interval to maximize ventricular filling without truncating the atrial contribution as observed by echocardiography.
Echocardiographic approaches provide only an open-loop method for optimizing CRT. After evaluating the hemodynamic effect of varying combinations of pacing intervals, a physician must manually select and program the desired parameters and assume that the patient's device optimal settings remain unchanged until a subsequent re-optimization visit. Automated systems for selecting pacing intervals during multi-chamber pacing have been proposed. A four-chamber pacing system that includes impedance sensing for determining the timing of right heart valve closure or right ventricular contraction and adjusting the timing of delivery of left ventricular pace pulses is generally disclosed in U.S. Pat. No. 6,223,082 issued to Bakels, et al., incorporated herein by reference in its entirety. Programmable coupling intervals selected so as to provide optimal hemodynamic benefit to the patient in an implantable multichamber cardiac stimulation device are generally disclosed in U.S. Pat. No. 6,473,645 issued to Levine, incorporated herein by reference in its entirety.
Doppler tissue imaging has been used clinically to evaluate myocardial shortening rates and strength of contraction. Myocardial acceleration during isovolumic contraction derived from tissue Doppler imaging has been investigated as an index or right ventricular contractility. Myocardial acceleration was presumed to be constant during the isovolumic contraction. Doppler tissue imaging has also been used to investigate coordination between septal and lateral wall motion for predicting which patients are likely to benefit from cardiac resynchronization therapy. Evidence suggests patient response is dependent on the degree of ventricular synchrony before and after therapy. Doppler tissue imaging studies have shown that the left ventricular mid to mid-basal segments show the greatest improvement in shortening following cardiac resynchronization therapy.
Detection and monitoring of left ventricular wall motion, therefore, would be useful in optimizing cardiac resynchronization therapy. Myocardial contractility is not as preload-dependent or autonomically sensitive as hemodynamic measures of ventricular function. Therefore optimization of cardiac resynchronization therapy based on myocardial contractility is expected to be less transient than optimization based on hemodynamic parameters, which could quickly change under autonomic influence or alterations in preload. Myocardial acceleration, however, is not a constant during isovolumic contraction when measured directly by an accelerometer. Therefore, a method is needed for monitoring myocardial acceleration, particularly in the left ventricle for use in assessing cardiac contractility and optimizing CRT.
Implantable sensors for monitoring heart wall motion have been described or implemented for use in relation to the right ventricle. A sensor implanted in the heart mass for monitoring heart function by monitoring the momentum or velocity of the heart mass is generally disclosed in U.S. Pat. No. 5,454,838 issued to Vallana et al. A catheter for insertion into the ventricle for monitoring cardiac contractility having an acceleration transducer at or proximate the catheter tip is generally disclosed in U.S. Pat. No. 6,077,236 issued to Cunningham. Implantable leads incorporating accelerometer-based cardiac wall motion sensors are generally disclosed in U.S. Pat. No. 5,628,777 issued to Moberg, et al. A device for sensing natural heart acceleration is generally disclosed in U.S. Pat. No. 5,693,075, issued to Plicchi, et al. A system for myocardial tensiometery including a tensiometric element disposed at a location subject to bending due to cardiac contractions is generally disclosed in U.S. Pat. No. 5,261,418 issued to Ferek-Petric et al. All of the above-cited patents are hereby incorporated herein by reference in their entirety.
Detection of peak endocardial wall motion in the apex of the right ventricle for optimizing A-V intervals has been validated clinically. A system and method for using cardiac wall motion sensor signals to provide hemodynamically optimal values for heart rate and AV interval are generally disclosed in U.S. Pat. No. 5,549,650 issued to Bornzin, et al., incorporated herein by reference in its entirety. A cardiac stimulating system designed to automatically optimize both the pacing mode and one or more pacing cycle parameters in a way that results in optimization of a cardiac performance parameter, including for example heart accelerations, is generally disclosed in U.S. Pat. No. 5,540,727, issued to Tockman, et al.
Optimization of both A-V intervals and V—V intervals during CRT can be a time-consuming process. Adjustment of one can affect the optimal setting for the other when the optimization is based on preload-dependent hemodynamic indices. It is desirable, therefore, to provide a method for optimizing V—V intervals based on a relatively preload-independent index of ventricular function, such as myocardial contractility, such that the optimization of the V—V interval is independent of the optimization of the A-V interval.
It is apparent from the above discussion that a need remains for providing a device and method for monitoring myocardial contractility in the left ventricle and for selecting optimal cardiac pacing intervals that produce the greatest improvement in left ventricular contractility during multi-chamber or biventricular pacing delivered to improve heart chamber synchronization, chronically or acutely. An improved index of left ventricular contractility is expected to reflect an improvement in overall cardiac chamber synchrony and function and generally result in a net improvement in cardiac efficiency.